Why did extensive flooding occur upstream of St. Louis during the 1993 Mississippi River flood?
A) Widespread urbanization in the upper portion of the basin increased runoff to a great degree.
B) Floodwalls protecting St. Louis caused a bottleneck in the river's flow.
C) The St. Louis floodwall failed, flooding St. Louis and the area upstream.
D) An intense thunderstorm dropped several inches of rain on the area in only a few hours.
B
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